The invention relates generally to an advantageous spool design, for example for a belt driven robotic actuator. In general, in belt driven actuators, an actuator exerts a tensile force via a belt that winds around a spool during actuation.
In one exemplary embodiment, the application of this device generally relates to exoskeletons, a device worn by a person to augment physical abilities, where mass and form factor are of high importance. It is desirable for exoskeletons to be as lightweight as possible, since the user must carry and move the device along with the body. It is also desirable for these devices to be capable of providing large amounts of force, torque and/or power to the human body in order to assist with motion. Furthermore, the device should not interfere with the natural range of motion of the body imposing additional device form factor and actuator constraints.
In passive exoskeletons, where there is no electrical control, it is desirable to have linear application of force/torque from mechanical systems so that the user experiences a smooth interaction with the mechanism. In active exoskeletons, where there is electronic control and/or actuation, it is desirable to have a linear application of force/torque. This force/torque must be repeatable and predictable to consistently model and control an exoskeleton system.
Since DC motors suited to use in wearable and/or mobile robotic applications have low toque output but high speeds, a high transmission ratio is needed to increase the torque of the mechanism. It is important to implement this transmission in a way that minimizes increase in mass while maintaining a suitable form factor. When designing a spool for this application, a spool that has a small diameter increases the transmission ratio. It also has a low mass and volume which reduces the size and mass of the assembly.
Wire rope or cord has been used effectively for similar applications, however, belts have certain advantages over wire ropes. A wire rope requires a consistent winding pattern that prevents the wire from tangling or jamming itself. This is accomplished with some sort of spooling guide that typically increases the complexity of the mechanism. The wire rope is also subject to tangling when tension is removed, therefore tension must be maintained either mechanically or operationally. Because of the flat nature of a belt, it can be wrapped around itself without tangling removing the need for a spooling guide. This also means the belt does not have to remain in tension which reduces mechanism complexity and/or reduces operational complexity.
When using a spool to transmit force the belt or cord will undergo curvature when winding. This tensile members in a cord are typically twisted or braided. This has the effect of different bend radii between the inner and outer fibers. To avoid rubbing of these fibers on one another a large spool diameter is required. Belts typically have tensile members in one plane reducing the need for a large diameter spool.
The unique requirements of the robotic exoskeletons including low mass, high force/torque/power, specific form factor, and linear application of force/torque create unique design challenges that prior art has not addressed.